1. Field of the Invention
The present invention relates to a developing unit and a developing method adapted to an image forming apparatus such as a copier, a facsimile machine, and a printer, as well as to the image forming apparatus incorporated with the developing unit or employing the developing method, and particularly to an arrangement for generating a developing bias voltage in a so-called hybrid developer of an electrophotographic system.
2. Description of the Related Art
The hybrid developer incorporated with dual rollers consisting of a magnetic roller and a developing roller has been conventionally employed as non-contact developing means in which a single component developing agent is used. In recent years, use of the hybrid developer as developing means in a tandem-color image forming apparatus in which a two component developing agent is used has been popular in light of the merit that toner images are relatively accurately superimposed one over the other. According to the conventional art, although deposition of a thin toner layer on the developing roller is feasible, it is difficult to scrape and feed the toner which has once been deposited on the developing roller back to the magnetic roller because the toner of high chargeability (charged amount) is electrically attracted to the developing roller, with the result that the toner cannot be sufficiently recovered merely with use of a magnetic brush formed on the magnetic roller. Thus, it is likely that drawbacks such as fog, ghost image, unwanted toner scattering, density distribution variation, and toner transfer failure may take place owing to toner residues which have been left on the developing roller.
In order to solve the above drawbacks, it is necessary to generate a strong alternate current (AC) field between the developing roller and the magnetic roller, or generate an electric potential between the developing roller and the magnetic roller by application of a high voltage of direct current (DC) to the developing roller during a non image formation period. In the above technique, however, application of a high DC voltage may adversely affect discharging operation onto a photosensitive drum or a charging distribution of toner over the developing roller due to instantaneous mixing of the toner.
Japanese Unexamined Patent Publication No. HEI 3-113474 proposes a so-called powder cloud development, as a measure for solving the above drawbacks. According to the method, a sub electrode including an electrode wire is provided between a developing roller and a photosensitive drum, and a toner cloud is generated by applying a weak AC field to the sub electrode. This method, however, is of less use, because the electrode wire of the sub electrode is likely to be smeared, and image formability may be degraded due to vibrations generated by the AC field.
Japanese Unexamined Patent Publication No. 2003-21966 proposes an arrangement in which toner circulation between a developing roller and a magnetic roller is accelerated by varying the duty ratio of the AC voltage to be applied to the developing roller while keeping the developing roller and the magnetic roller in an equipotential state. A large amplitude of the AC voltage is required in order to carry out satisfactory toner circulation between the developing roller and the magnetic roller, which may influence the physical deterioration on the photosensitive drum.
There have been proposed various arrangements such as a special structure for generating an AC field, and a method for controlling an application voltage, other than the above. These arrangements, however, make the construction of a developing unit complicated, and raise the cost relating to a developing bias voltage generator.
As mentioned above, the respective conventional arrangements have been proposed in an attempt to prevent deterioration on image density or developing performance. In these conventional arrangements, complicated parts or a large number of parts are required. In addition to the above, control of the developing bias voltage is cumbersome. Thus, the cost relating to the developing unit including the developing bias voltage generator is unavoidably raised.